(a) Field of the Invention
The present invention relates to a method for resolving collisions in communication systems using medium access control based on contention, where the media access control is used to reserve or require communication resources in a mobile terminal. More specifically, the present invention relates to a method for resolving collisions.
(b) Description of the Related Art
In a mobile communication system, a terminal reserves communication resources or requests the same from a base station or a remote host (also referred to as “base station” hereinafter). To support this function, the base station allocates mini slots and broadcasts the number and the location of the mini slots to all the terminals. In this case, each of the terminals competitively attempts to access the mini slots by using a media access control method. At this time, when the number of terminals that are accessing a certain mini slot is two or more, a collision arises in the mini slot and the access attempt results in failure. Therefore, the base station inspects whether a collision has arisen in the mini slots and broadcasts the inspected information to all the terminals by incorporating the inspected information into a header of a frame.
In such a media access control method, a prior art called a BEB (Binary Exponential Back-off) method is known to resolve a collision. The BEB method is performed in all the terminals. Each terminal receives broadcasting information incorporated in the header of a present frame and confirms through the broadcasting information whether an access attempted in a previous frame into a certain mini slot has been successful. According to the BEB method, when a certain terminal continues to fail in access attempts, the number of access attempts shall be assumed to be an integer ‘a’. In these circumstances, when 2a is less than or equal to the number of mini slots (N), any integer value k is selected by using a uniform distribution from 1 to N. Here, the integer value k is less than or equal to the value allowable in the communication system. When 2a is larger than the number of mini slots (N), the integer value k is selected by using a uniform distribution from 1 to 2a. Then, a count operation of mini slots starts at a current frame, and the kth mini slot is accessed.
In the above BEB method, when the re-access number of the terminals increases due to a collision, access of the terminals to the mini slots can be dispersed by delaying a re-access opportunity of the corresponding terminal until the count operation started at any mini slot of a current frame arrives at the kth mini slot. This results in resolution of collisions. However, this method has a problem in that as more attempts at re-access occur, the time to wait for access increases. Therefore, there is an opportunity for access priority to be given to a terminal that has attempted access, but this leads to an inequality problem. Further, in the case that many terminals attempt to access, the possibility of collision increases even though the access attempts of the terminals have been dispersed by using the BEB method.
A method using a contention window has been known as another attempt to resolve the above-described collision in the communication system. In the method using a contention window, a base station incorporates the number of mini slots and a variable contention window size (referred to as ‘CW’ hereinafter) into a header of a frame, and broadcasts the header to each of the terminals. Each of the terminals selects a certain integer k from the constant distribution between 1 to CW, and when k is less than or equal to N, all terminals can access their kth mini slot. Otherwise, the terminal delays access of the mini slots to the next frame. This method can solve the inequality problem because all the terminals can perform the access operation equally. However, the method still has a problem in that the greater the number of terminal access attempts, the more the possibility of collision increases.
Meanwhile, a relevant prior art of application No. 10-1996-0053728 was filed with the Korean Intellectual Property Office under the title “A multiple access control method in a radio data communication network” on Nov. 13, 1996. In the prior art, a transmission medium is accessed by mixing reservation and contention to access communication resources in accordance with various traffic characteristics, and to thereby increase channel efficiency in radio communication. However, the prior art relates to a method of controlling medium access by mixing reservation and contention techniques, while the present invention relates to a method for reducing collision in a communication system using medium access control based on contention. Therefore, the present invention has a different technical purpose and solution from the prior art.